This invention relates to graphic recording apparatus, more particularly a cathode ray line scan recorder.
In this type of device the light recording medium (e.g. light sensitive film or paper) is moved by mechanical drive rollers, along a path substantially normal to the display medium (e.g. a CRT). The image is recorded on the moving paper as it is exposed by a beam of light from the display medium as a gray scale recorded image.
In many graphic recording devices of this type, the quality and fidelity of this recorded image is dependent upon the constancy of the velocity of the recording medium along its path of travel. The display medium is typically designed to expose the recording medium, a line at a time, at a constant rate based upon an average velocity of the recording medium. Due to the mechanical linkage for moving the recording medium, the velocity of the recording medium is non-uniform. This non-uniformity of motion of the recording medium results in uneven exposure of the recording medium. When the motion of the recording medium is faster, the recorded lines are spaced slightly further apart and the recorded image appears to be lighter; when the motion of the recording medium is slower, the line spacing is closer and the recorded image appears to be darker.
An analog approach to this problem has been disclosed in U.S. Pat. No. 4,172,259. The technique discussed therein requires the constant monitoring of the velocity of the recording by generating a signal representative of that velocity, applying that analog signal to a frequency to voltage converter, differentiating that voltage signal, integrating the differentiator output, and modulating the vertical deflection circuit of the CRT in a compensating direction relative the base or average velocity position as determined by the error signal.
A technique that is digital and only addresses the horizontal scan modulation of the CRT beam would simplify the circuit and the volume of space necessary to properly control the operation of the recorder. It is believed that the present invention is such a device.